Added --pkg-path and --gen-pkg-only flags.
Added --pkg-path and --gen-pkg-only flags.

This involved some re-jiggering of the `cache_action_for` and `compile`
functions.  Short version: knowledge on what to do is now more centralised
(into an `InputAction`).  `cache_action_for` -> `decide_action_for` and
`compile` -> `gen_pkg_and_compile`.

DanielKeep/cargo-script,durka/cargo-script,durka/cargo-script